Manufacture yield and rage of use by product users (power supply and temperature) for integrated circuits is enhanced by an ability to generate voltages that are relatively invariant with variation in power supply, temperature, and process. Evolution of improved art has included diode or zener clamps driven by a resistor, and then by current source to reduce variation in the current through the diodes. See Chapter 20 and 23 in CMOS Circuit Design, Layout and Simulation by Dr. R. Jacob Baker, 2nd Edition, which is herein incorporated by reference herein in its entirety. While such circuits were often better than a resistor divider, the variation with temperature and even power supply were still substantial. These were further improved with the Widlar bandgap reference. See write-up about use of bipolar bandgap to create stable reference in U.S. Pat. No. 5,053,640, which is hereby incorporated by reference herein in its entirety. One conventional approach to providing a voltage reference has been to use temperature compensated zener diodes. Since the breakdown voltage of a zener diode is about 6 volts, however, this provides a lower limit on the input voltage employed in a voltage regulator circuit. Other disadvantages are also associated with zener diode voltage references, such as stability problems, process control problems and noise introduced into the circuit.
In another approach, the bandgap voltage of silicon is employed as an internal reference to provide a regulated output voltage. This approach overcomes many of the limitations of zener diode voltage references such as long-term stability errors and incompatibility with low voltage supplies. One such convention bandgap voltage reference is disclosed in R. Widlar, New Developments in IC Voltage Regulators, IEEE J. Solid-State Circuits, Vol. SC-6 (February 1971), which is hereby incorporated by reference herein in its entirety, and is illustrated generally in FIG. 1. In this approach, a relatively stable voltage is established by adding together two scaled voltages having positive and negative temperature coefficients, respectively. The positive temperature coefficient is provided by the difference between the base-emitter voltages of two bipolar transistors Q1 and Q2 operating at different emitter current densities (referring to FIG. 1). Since these two transistors are operated at different current densities, a differential in the emitter-base voltages of the two devices is created and appears across R3. The negative temperature coefficient is that of the base-emitter junction of transistor Q3. Thus the basic bandgap cell requires three transistors, Q1, Q2 and Q3 to achieve the offsetting temperature coefficients. It can be shown that, for theoretically perfect device operation, if the sum of the initial base-emitter voltage of Q1 and the base-emitter voltage differential of the two transistors Q1 and Q2 is made equal to the extrapolated energy bandgap voltage, which is +1.205V for silicon at T=0° K, then the resultant temperature coefficient equals zero. (The detailed derivation of this result may be found in the above-noted Widlar reference.)
However this approach uses bipolar devices, a process limitation for use with MOS and FET processes. Translating using beta-multiplier is shown in Dr. Baker's book, FIG. 23.13 with results at FIG. 23.13. And trimming means are described as are familiar to those reasonably skilled in the art. Using parasitic bipolar junction transistors in the MOS process allows approximating band-gap operation with good results. Example circuits using the parasitic bipolar devices and results are shown in FIG. 23.25 and FIG. 23.26. These approaches results in good references and are suitable for use in regulators. However, this approach requires providing the extra process step of the n-well (commonly associated with the CMOS or bi-cmos processes).
For high volume memory, especially cost sensitive commodity memory such as a Flash replacement, it is desirable to find way without these extra process steps or special transistor requirements to generate a reference. Such a preference should preferably have adequate performance to allow regulating internal nodes on the chip, such as the write voltage, by determining whether when the charge pump should be turned on and off to control the voltage generates that is above or below the power supply. And the reference can desirably be used as an input to a comparator for determining whether inputs to the chip are logic as 1s or 0s. And a regulator can be useful in the sense amp to determine memory state of signals from the memory array. Other uses may also be found by those reasonably familiar with the art for a reference and regulator generates by a lower cost process with fewer masks and process steps.